Tools for cutting holes in various materials such as sheet metal generally fall into two types, those which operate by cutting or removing the entire interior area of a circle and those which operate by cutting only the circumferential area of a circle thereby producing an interior circular plug during the cutting operation. A conventional fluted drill is an example of the former type of tool. Such tools have the disadvantage of requiring substantially more cutting activity in order to produce the desired hole. Over a certain diameter, it is much more efficient and generally quicker to utilize a tool which cuts a thin circumferential circle for removing the metal as a plug. Such tools are generally known as trepanning tools. Tools of this type require substantially less cutting to achieve a desired hole and therefore provide large diameter holes quickly and with substantially less energy consumption.
The use of trepanning tools which cut only the circumferential area of circles to provide large diameter holes is widely known. Such tools generally comprise an elongated tubular cutting head having cutting blades or tips secured thereto which engage a work piece to shave or cut portions thereof to define the circumference of a circle upon rotation of the tool. Accordingly, as cutting of a work piece is effected, a center plug is produced which is received in the center area of the elongated tubular cutting head. Various ejector devices are commonly used with such tools to remove the plug from the tool, such as spring loaded arms. Tools of this type have been available for cutting holes having diameters ranging from approximately less than an inch to holes having extremely large diameters for example up to 10 inches or more. Such large diameter trepanning tools are generally associated with lathe type machines which are fixedly mounted for producing holes in an automated fashion. Generally, such trepanning tools are therefore rigidly aligned with the material to be cut which is usually also clamped or fixed to prevent movement. While such applications are well known in the art, there has been a continuing effort to produce tools suitable for use in a hand held fashion which will provide a relatively precise and smooth hole in thin sheet metal for use, for example, in field construction such as by an electrician in cutting holes in electrical boxes. In addition, it is desirable to cut holes in other types of sheet metal such as stainless steel, which is finding more use in home and building construction.
Various efforts have been made to produce trepanning tools which can be effective in cutting holes in sheet metal in diameters from approximately 1/2 inch up to 2 inches. However, such efforts have been generally unsatisfactory, as it is difficult to align hand held tools for producing smooth round holes. In addition to the loss of rigidity, hand held drills have torque and RPM limits considerably less than those available in a fixed machine structure. Consequently, a trepanning tool for hand held applications must produce holes with a minimum of resistance to preclude stalling or binding.
Most of the tools produced to date have suffered from rapid wear which requires frequent replacement of either the entire tool or of the cutting tips attached thereto. For example, in U.S. Pat. No. 4,490,080 to Kezran, there is disclosed a hole cutting tool which includes removable cutting tips. Kezran relies on means which retain the tips in slots comprising a retaining wedge which engages the leading edge of the slot and engages the leading edge of the tip to urge it against the trailing edge of the slot so that the cutting tip is wedged in position with the angle of the wedge and the angle of the trailing edge of the slot preventing outward movement of the tip. While such a trepanning tool may be effective in various applications, where field use is complicated it is difficult to provide for tip replacement as such tips are small and easily lost. Should a trepanning tool with dull tips be used, it would require additional torque and undergo accelerated wear, and produce a ragged hole due to binding of the tips in the sheet metal.
Generally, where multi-tip tools are used, wear may be uneven, causing binding to occur through misalignment of one or more tips within the hole Such binding is frustrating for the tool operator, and difficult to control with a hand held driving device, requiring additional manual force to overcome; Such a problem is additionally time consuming and often produces a ragged hole. Consequently, the search continues for a trepanning tool which is substantially wear resistant, and capable of producing smooth holes without binding in field applications using a hand held drill and to do so without requiring frequent replacement.